1. Field of the Invention
The present invention relates to methods and devices intended to provide assistance in the driving of motor vehicles, particularly when the external brightness is low, making it necessary to switch on low beam headlights (also called “statutory” lights).
2. Description the Related Art
The light beams emitted by lighting devices fitted to vehicles are governed by international regulations which specify the maximum and minimum intensities to be used, for example as they appear on a screen placed at a distance from and in the axis of the lighting devices. In the specific case of low beam headlights, the aim of these regulations is simultaneously                to enable the driver of the vehicle fitted with this lighting device to have a satisfactory illumination of the road scene on which he is traveling, so that he can perceive his environment in the best possible conditions, and        to avoid dazzling the drivers of other vehicles, regardless of whether they are traveling in the opposite direction (approaching vehicles) or in the same direction (followed vehicles).        
If the low beam headlights of an approaching vehicle are switched on and the headlights emitting the low beam are poorly adjusted or even slightly out of adjustment, this will be enough to cause dazzling, since the low beam of the approaching vehicle will include light rays higher than an upper regulatory limit called the cut-off level, these rays being prohibited by the regulations.
Moreover, even if the lighting devices of the approaching vehicles are correctly adjusted, the emitted beam may still cause dazzling if the outer lenses of these devices, through which the light rays pass, are soiled. This is because any soiling present on the outer lens forms centers of light diffusion, that is to say secondary light sources, which emit light in all directions. The greater the soiling of the outer lens, the greater is the tendency for the lighting device to emit a dazzling beam.
The low beam headlights of approaching vehicles may also be dazzling in another situation, in which the rear trunks of these vehicles contain relatively heavy luggage. In this case, the attitude of the vehicle is changed and ceases to be horizontal, the front of the vehicle being raised. Manually or automatically operated correctors are normally provided to lower the light beam to bring it back into conformity with the regulations. If the corrector has not been operated or if it is faulty, the low beam will include light rays located above the upper regulatory limit, these rays causing dazzling and being prohibited by the regulations.
These risks of dazzling by a vehicle approaching in the opposite direction are increased if the vehicle is a heavy goods vehicle, the headlights of which are generally fitted higher up than those of light vehicles.
Various solutions have been proposed to mitigate this situation.
For example, the document FR 2 846 756 discloses a method for improving the night vision of drivers, including a first polarizing filter in the vicinity of the light sources and a second polarizing filter having a polarization direction perpendicular to that of the first filter, in the field of view of the drivers.
This solution is theoretically satisfactory, but in order to be effective it would require all motor vehicles on the road to be fitted with polarizing filters on the protective outer lenses of their lighting devices, and would also require the driver to be provided with polarizing glasses.
Additionally, EP 0 498 143 A1, which is equivalent to U.S. Pat. No. 5,258,607, discloses an anti-dazzle device including an optical sensor supplying a proportional signal at the value of the ambient brightness within a predetermined measurement angle. The sensor transmits this signal to a circuit for measurement and comparison with a predetermined threshold value. If the measured value exceeds the predetermined value, the comparison circuit sends a signal for switching on an electrosensitive screen which is designed to change from a state of total transparency in the absence of a signal to a partially transparent or colored state when the signal is transmitted, and then to return to the transparent state when the signal ceases.
The drawback of this device is that it affects the perception of the whole road scene. If the ambient brightness exceeds the authorized threshold, the whole road scene will be obscured, although the excess brightness may be created by only one element forming a single point in the road scene. In this case, a driver provided with this anti-dazzle device may fail to see certain important elements in the road scene, such as a traffic sign warning of an imminent hazard, or the road surface illuminated by his own headlight beam.
There is a known method and device, for example that disclosed in WO 96/20846, for attenuating the light from the headlights of approaching vehicles, comprising the emission of light pulses by the vehicle headlights and the control of the transparency of filters placed before the driver's eyes in synchronization with the light pulses emitted by the headlights of the vehicle, the transparency of the filters being at a maximum for a duration exceeding that of the light pulses. In this way the brightness of oncoming headlights is reduced.
U.S. Pat. No. 5,486,938 also discloses an anti-dazzle system for drivers, comprising a generator which controls, in a synchronized manner, headlights fitted with flashlamps and a liquid crystal screen, placed before the driver's eyes, in such a way that the transmission of the screen is maximal during the emission of the light pulses.
The last two of these documents describe systems which are cumbersome and difficult to use, which are relatively slow in operation with relatively long response times, and in which the transparency of the filters or screens used is always less than 50%; that is to say, these systems, using liquid crystals, cause a decrease in the perceived luminous intensity, even when their transmission is maximal.